Opsonic requirements for several species of bacteria will be studied using several serum sources. A primary objective is evaluation of the role of specific immunoglobulin antibodies, the classical complement pathway, and the alternative complement pathway in opsonization of bacteria that produce disease. Opsonic sources will include normal human serum, heated serum, C2 and C4 deficient serum, immunoglobulin deficient serum and MgEGTA chelated serum. The basic assay system will be evaluation of leukocyte-association of radiolabeled bacteria. This method allows study of the kinetics of uptake of bacteria by phagocytic cells and also allows precise quantitation of opsonic requirements. The critical factors on the surface of S. aureus, E. coli, Streptococcus pyogenes and S. pneumoniae which require opsonization before phagocytosis will also be studied. Preliminary evidence suggests that cell wall peptidoglycan requires opsonization before efficient phagocytosis occurs. Complement functions as an excellent opsonin for peptidoglycan. In certain highly pathogenic strains of certain species (encapsulated S. aureus, K1 E. coli and M protein positive Strep. pyogenes) specific antibody is required to neutralize the anti-opsonic effect of capsular factors.